It is said that ultraviolet rays may become causes of not only suntan or sunburn but also aging phenomena or skin cancer. Therefore, ultraviolet-shielding agents are widely used in the cosmetics field.
For example, as ultraviolet-shielding agents, organic ultraviolet absorbers such as benzophenone-based absorbers, methoxycinnamic acid-based absorbers, and dibenzoyl methane-based absorbers or inorganic ultraviolet-scattering agents such as zinc oxide and titanium oxide are generally used.
However, for organic ultraviolet absorbers, there are problems of a concern that organic ultraviolet absorbers may deteriorate due to heat or irradiation with ultraviolet rays for a long period of time and a necessity of the combined use of multiple kinds of organic ultraviolet absorber having different ultraviolet-absorbing wavelengths since it is not possible to absorb a wide range of ultraviolet rays with only one kind of organic ultraviolet absorber.
On the other hand, inorganic ultraviolet-scattering agents have advantages that inorganic ultraviolet-scattering agents do not deteriorate due to heat or irradiation with ultraviolet rays for a long period of time, are excellent in terms of weather resistance, heat resistance, and the like, and are capable of scattering ultraviolet rays in a wide wavelength range. However, for inorganic ultraviolet-scattering agents having the above-described advantages as well, there are problems as described below. That is, inorganic ultraviolet-scattering agents scatter not only ultraviolet rays but also visible light rays, and thus, when a large amount of the inorganic ultraviolet-scattering agents are formulated into cosmetics or the like, there is a problem in that the face is likely to be white. Therefore, in order to cope with the above-described problem, the inorganic ultraviolet-scattering agents are used in a form of being appropriately combined with organic ultraviolet absorbers.
Recently, water-based cosmetics have been being used particularly as a variety of cosmetics such as sunscreens, emulsions, and creams since water-based cosmetics are not sticky and provide a silky feeling compared with oil-based cosmetics. In the above-described cosmetics, titanium oxide and zinc oxide are mainly used as inorganic ultraviolet-scattering agents.
Meanwhile, zinc oxide has photocatalytic activity. Therefore, when zinc oxide is used in cosmetics, troublesomely, it is necessary to strictly select the amount of zinc oxide formulated or other components formulated with zinc oxide. Therefore, for the purpose of suppressing the photocatalytic activity of zinc oxide, there is proposed surface-coated zinc oxide in which the surface of zinc oxide is coated with a low-active substance such as silicon oxide or aluminum oxide (for example, refer to Patent Literature 1 and 2).
Meanwhile, in a case in which zinc oxide is used in water-based cosmetics, intrinsic problems of water-based cosmetics are caused. That is, since zinc oxide is an oxide of an ampholytic metal, zinc oxide readily dissolves in acids and alkalis and also dissolves slightly in water and, consequently, discharges zinc ions. Therefore, in a case in which zinc oxide is used in water-based cosmetics, there has been a concern that eluting zinc ions may react with water-soluble polymers and the like of organic ultraviolet absorbers and viscosity improvers and problems of the degradation of cosmetic performance, changes in color and viscosity, and the like may be caused. That is, there has been a problem in that the degree of freedom in terms of formulation is restricted.
Particularly, when carbomers such as carboxy vinyl polymers which are ordinarily used as viscosity improvers and zinc oxide are jointly used, there is a problem in that eluting zinc ions and carboxylate groups (COO—) react with each other, and thus the gel structures of the carbomers are broken, and the viscosity decreases. Therefore, in order to suppress the elution of zinc ions, there is proposed surface-coated zinc oxide in which the surface is coated with an inorganic compound such as silicon oxide (for example, refer to Patent Literature 3 and 4).